The use of piezo ceramic acoustic horns in warning devices has been suggested to simplify manufacture and improve reliability. A smoke detector for example, may require an 85 decibel output from a 9.1 volt power supply. In order to achieve this output level, it is necessary to operate the piezo horn at its resonant frequency.
Various methods have been tried in order to drive a piezo horn at its resonant frequency to maximize sound output. Accuracy is required, however, since the anti-resonant frequency of the horn is normally within 300 to 500 hertz of the resonant frequency, and thus requires precision in adjustment of the tuning circuit to the horn.
Various methods have been suggested utilizing a feedback terminal which has a peak output when oscillating on the resonant frequency. This circuit has two major problem areas for use in a battery operated device, for example. This system in operation tends to be power consumptive and therefore impractical for battery operation. Another problem is that the supply voltage is generally insufficient to get an 85 decibel sound output required for example, in a smoke detector, from the piezo horn.
The voltage may be doubled by alternatively switching the positive side of the voltage supply and the ground connection between the two terminals of the piezo horn and thus the total voltage swing of each side of the horn will be twice the total voltage supply value. This method effectively increases the power to the piezo horn, however, this method still requires the feedback detector and circuitry required to sense the proper switching and the circuitry required to initiate the resonant oscillation. Furthermore, it is difficult to get the feedback voltage signal to operate precisely in phase with the actual resonant signal.
Accordingly, it is an object of the present invention to provide a piezo horn driver circuit which is self-starting without requiring external components.
It is a further object of the present invention to provide a piezo driver circuit that runs on resonant frequency without requiring a separate feedback signal.
A yet further object is to provide a piezo horn driver circuit having maximum voltage swing applied to the horn in a bidirectional oscillation.
A still further object is to provide a piezo horn driver circuit capable of interfacing with logic signals at low current levels.